gcanti / logging-ts

Composable loggers for TypeScript

Adapted from purescript-logging

fp-ts and TypeScript compatibility

logging-ts version	required fp-ts version	required TypeScript version
0.3.0	2.0.0+	3.5+
0.2.0	1.7.0+	2.8.0+

Usage

From purescript-logging's README

A logger receives records and potentially performs some effects. You can create a logger from any function (a: A) => HKT<M, void> for any A and M.

Unlike most other logging libraries, logging-ts has no separate concepts "loggers" and "handlers". Instead, loggers can be composed into larger loggers using the Semigroup instance. Loggers can also be transformed using contramap (for transforming records) and filter (for filtering records). An example use case might be the following:

import * as C from 'fp-ts/lib/Console'
import * as D from 'fp-ts/lib/Date'
import { chain, IO } from 'fp-ts/lib/IO'
import { pipe } from 'fp-ts/lib/pipeable'
import * as L from '../src/IO'

type Level = 'Debug' | 'Info' | 'Warning' | 'Error'

interface Entry {
  message: string
  time: Date
  level: Level
}

function showEntry(entry: Entry): string {
  return `[${entry.level}] ${entry.time.toLocaleString()} ${entry.message}`
}

function getLoggerEntry(prefix: string): L.LoggerIO<Entry> {
  return entry => C.log(`${prefix}: ${showEntry(entry)}`)
}

const debugLogger = L.filter(getLoggerEntry('debug.log'), e => e.level === 'Debug')
const productionLogger = L.filter(getLoggerEntry('production.log'), e => e.level !== 'Debug')
const logger = L.getMonoid<Entry>().concat(debugLogger, productionLogger)

const info = (message: string) => (time: Date): IO<void> => logger({ message, time, level: 'Info' })
const debug = (message: string) => (time: Date): IO<void> => logger({ message, time, level: 'Debug' })

const program = pipe(
  D.create,
  chain(info('boot')),
  chain(() => D.create),
  chain(debug('Hello!'))
)

program()
/*
production.log: [Info] 10/4/2019, 12:44:48 PM boot
debug.log: [Debug] 10/4/2019, 12:44:48 PM Hello!
*/

Documentation

• API Reference